It is known that the temperature of the glass sheet should be regulated precisely in both tempering and bending operations if products having constant characteristics are to be obtained. Various regulating systems have been developed, with the objective of obtaining the best possible characteristics of constancy of the glass sheet at the furnace exit. Normally, the glass sheets, suspended by self-locking clamps from a mobile carriage and moved in successive advances through a furnace, during tempering are heated within the furnace to a required temperature. Bending according to practiced techniques may be carried out before tempering. Such an overall process, that is, a process including bending and/or tempering, may be followed in the production of automotive window.
According to the prior art, it is known to provide a thermocouple to both respond to an electric heating device and provide a data signal to a regulator to maintain the temperature of the heating device(s), and the furnace, relatively constant (FR 794,398, DE AS 1,106,039). It is also known that the cells of a furnace, and even separate zones of the same cell may be heated according to a different program (DE OS 2,837,985).
It has also been proposed in the regulation of multicell furnaces wherein the glass sheets are suspended from conveying carriages and subject to interrupted advance to regulate the temperature of the heating elements to a predetermined value and use the measurement of temperature of the glass sheet as it exits the furnace as a regulation factor (U.S. Pat. No. 3,744,985).
In another process of the prior art individual cells of a furnace may be provided with a constant electric power input and, if the furnace temperature should deviate from a reference value, the rate of movement of each glass sheet through the furnace will be corrected (DE AS 1,596,382).
It is also known in the prior art to use an optical pyrometer at the furnace exit to measure the temperature of glass sheets, oriented in a horizontal position for movement, and regulate the rate of advance of the glass sheets through the furnace as a function of any deviation in temperature of the glass sheet at the exit from a desired value (DE OS 2,746,301). In this process, a microprocessor provides regulation control.
Finally, the prior art describes the use of thyristor units to regulate a power input (DD PS 78 780).
Many of the known systems function satisfactorily in a fabrication process, particularly in situations that the glass sheets of the same size, and of the same mass, are moved through a furnace at a constant rate and under established conditions of balance of an approximate equilibrium of furnace energy. Many of these furnaces comprise massive structures providing satisfactory insulation capability and strong thermal inertia, resulting in a good heat stability. Advantageously, these furnaces are not extremely sensitive to outside disturbances; on the other hand, these furnaces are not able satisfactorily to rapidly rebalance disturbances of operating conditions. Under the influence of the regulating system, the average temperature undergoes slight oscillation, but at a lower frequency than the rate of advance of the glass sheets through the furnace. Consequently, when temperature measurements are made inside the furnace, the temperature modifications do not immediately affect the glass sheets being treated. Rather, the effects of the temperature modifications are recognized after a considerable delay.
Further, it has been found the particular regulation difficulties arise under the above-noted circumstances, as, for example, under circumstances of disturbance of the energy balance and in situations of difficulties which may be linked to variances in the rate of advance of the carriage or in the regularity of advance of the glass sheets. Further, regulation difficulties may be experienced if one or more glass sheets release from the support structure so that the carriage, in its cycle, follows the path through the furnace either empty or with a number of glass sheets less than the number originally provided.
In the prior art, it has been impossible or substantially difficult to regulate the heating power in the furnace cells, if a gap occurs in the conveying system, so that the glass sheets that follow immediately after the vacant spot are heated to a satisfactory temperature. In general, several defective glass sheets will be found to have exited the furnace before it is able to regain balance.